Relief mapping
A per-pixel shader technique that fakes real surface depth by ray-marching a height map, so bricks and cobbles occlude one another and show correct parallax at grazing angles.
Introduced by Policarpo, Oliveira and Comba in 2005 as an extension of parallax mapping: instead of a single offset lookup, the fragment shader ray-marches through a depth/height texture to find where the view ray intersects the stored surface. The result gives self-occlusion, self-shadowing and correct silhouettes — all on flat geometry, at a fraction of the triangles real displacement would need.
It trades vertices for texture samples, so cost scales with screen resolution rather than mesh density. The cheaper cousins are normal mapping (no depth) and parallax mapping (one step); true displacement mapping moves actual geometry.